This isn't a huge deal, really, but it's a major peeve of mine, nonetheless. In the life of every computer with WMP 11 on it that I've owned, this has happened with at least one category or another in the library. This piece of a screenshot shows the "Year" list display, a current example of this problem, where identically tagged files refused to be grouped together under one tag. Anyone familiar with this problem and/or know how to permanently fix it?
Have you tried going to the individual files in Windows Explorer (I'm presuming you use Windows), erasing the year fields and then tagging them again from there? It happens to me very occasionally and that's what I do to fix it.
This happens to me in Foobar sometimes; might be a similar issue. The reason it doesn't get grouped because there's one tag that doesn't *exactly* match with the others, whether it's the album name, artist name, year, or maybe even bitrate. I somehow doubt it's the bitrate [say you have a bunch of 320 songs with one V2 mixed in there, that'll ungroup it], but it could be. Watch out for random spaces too.